1. Field of the Invention
This invention relates to a method of liquefying and fractionating air and, more particularly, to a method of fractionating air in a total low-pressure system to economically produce oxygen of high purity.
2. Description of the Prior Art
Air liquefaction-fractionation systems involving the liquefaction of air and the fractionation thereof into N.sub.2, O.sub.2, Ar, etc. are in operation in a broad range of industrial fields. In these air liquefaction-fractionation systems, the material air and the product oxygen must be subjected to compression and decompression procedures under operating conditions and, therefore, the systems must incorporate compressors, expanders and other units. Regarding the power requirement of such machines, the power necessary for compression accounts for a major proportion of the energy used. In fact, the compressor accounts for more than half of the power cost requirement of such an air liquefaction-fractionation plant. Since many air liquefaction-fractionation systems are of large capacity and involve high utility costs, a decrease in power consumption has been in keen demand for the purpose of reducing the overall production cost of oxygen. Such a situation holds true for the so-called total low-pressure air fractionation plant. Nonetheless, substantially no effective measures have been taken in this regard as yet.
The conventional total low-pressure air fractionation system generally operates on the principle depicted in the flow chart of FIG. 1. Thus, the material air supplied through an air filter 1 is pressurized to about 4.6 ata in an air compressor 2. The compressed air is then cooled by an after-cooler 3 and enters a reversible heat exchanger 4 where it undergoes heat exchange with product oxygen and impure nitrogen, whereby it is cooled down to near its boiling point. This air is further fed into first condenser at the bottom of a low-pressure fractionation column 5 (hereinafter referred to briefly as the upper column) and is super-cooled in this condenser 6 to less than its boiling point by heat exchange with the reflux liquid in the upper column 5, whereby it is partially liquefied. Then, in a gas-liquid separator 7, gaseous air and liquefied air are separated from each other and the liquefied air is guided into a total fractionation medium-pressure column 8 (hereinafter referred to briefly as the lower column). The liquefied air in the lower column 8 is gasified to form an ascending gas stream and brought into contact with a reflux liquid (nitrogen-rich liquid) formed by condensation in the top portion of the lower column 8, whereby it is crudely fractionated so that while a nitrogen-rich gas is produced in the top portion of the lower column 8, and said reflux liquid becomes liquefied air containing about 40% of oxygen in the bottom portion of the lower column 8. The gaseous air withdrawn from an intermediate stage of the lower column 8 enters an expansion turbine 9 via conduits 31 and 32, where the required refrigeration is generated. From the turbine 9 the air is passed into the upper column 5 via a conduit 33.
The liquefied air which has undergone crude fractionation in the lower column 8 as mentioned above is fed through a conduit 34 to a liquefied air super-cooler 10 in which it is cooled. The cooled liquefied air is guided via a conduit 35 to a second condenser 12 disposed in the top portion of the crude argon column 11, and in this condenser 12 undergoes that exchange with argon-rich gas in the crude argon column 11, after which it is guided via a conduit 36 to the uppe column 5. On the other hand, the nitrogen-rich liquid collected in the top portion of the lower column 8 flows through a conduit 37 into the liquefied air super-cooler 10 where it is cooled. The cooled air flows via a conduit 38 to the upper zone of the upper column 5. The gaseous air separated by the gas-liquid separator 7 flows through a conduit 27 and a liquefier 13, in which course it is entirely liquefied. The air so liquefied flows via a conduit 29, the liquefied air super-cooler 10 and a conduit 30 to the upper zone of the upper column 5. The oxygen-rich liquid fractionated in the upper column 5 and collected in the bottom zone is withdrawn from the lowermost end of the bottom portion of the upper column 5 and flows via a conduit 51 to a lower portion of the crude argon column 11 for further fractionation. Thereafter, the oxygen-rich gas of high purity in the bottom of the crude argon column 11 is withdrawn via a conduit 45 and fed to the liquefier 13 via a conduit 46. The oxygen-rich gas which has shown some temperature recovery in this liquefier 13 flows through a conduit 47 to the reversible heat exchanger 4 in which a major temperature recovery takes place. The gas is then fed via a conduit 48 to an oxygen compressor 14, where it is pressurized and recovered from the system as product oxygen.
In order to accomplish a smooth recovery of high-purity oxygen by the above-described total low-pressure air fractionation method, it is necessary that (1) the internal pressure of the crude argon column be maintained at a reduced pressure between about 0.8 ata and about 1.0 ata and (2) the pressure of product oxygen at recovery be adjusted to about 1.0 ata to 1.2 ata. For this purpose, the conventional total low-pressure air fractionation system includes an oxygen compressor 14 located in a conduit 48 for withdrawal of product oxygen emerging from the reversible heat exchanger 4 to decompress the crude argon column 11 by suction and to thereby meet the above requirement (1). Moreover, at the stage where a pressure drop to about 0.7 ata takes place at a reversible heat-exchanger 4 after withdrawal at about 0.8 to 1.0 ata from the bottom of the crude argon column 11, the product oxygen is compressed to about 1.2 ata so as to satisfy the above requirement (2). Since a large-scale oxygen compressor is used to effect such a comparatively small decrease and increase of pressure, the high power cost cannot be reduced effectively and acts as a rate-determining factor in the improvement of the overall economics of the air fractionation plant. Therefore, in total low-pressure air fractionation systems, it has been considered necessary to develop technology which would be capable of meeting the above-mentioned two operation requirements (1) and (2) under a minimum power cost for meeting these requirements.